The present invention relates to Time Division Multiplexed (TDM) communications. In particular, the present invention relates to techniques that increase the amount of information transmitted in a time slot by eliminating overhead information.
Modern communications networks carry vast amounts of information. The information is often transmitted serially in a Time Division Multiplexed (TDM) format in which information to or from many terminals (for example, cellular phones) is transmitted in a single channel (implemented, for example as a 30 Khz wide frequency band allocated from a larger spectrum). The time during which a given terminal may transmit (or receive) is generally referred to as a time slot. The duration of a time slot may, for example, be measured in bit periods, symbols, or chips.
Additionally, a satellite that forwards information to ground based users in a downlink beam may serially transmit the information in a time slot structure. Thus, a single transmitting source may also use a TDM format for transmitting data. Individual ground based terminals monitor the downlink beam, wait for the appropriate time slot, and extract their data. Systems that transmit information in time slots are generally referred to as Time Division Multiple Access (TDMA) systems.
In a TDMA system, bursts of data are transmitted in each time slot and there are a predefined number of time slots per frame. For example, the Telecommunications Industry Association (TIA) IS-54-B standard defines a six time slot frame. Each time slot is 162 symbols in length. An important consideration in a TDMA system is the alignment or synchronization of bursts so that overlap between transmissions (and therefore corrupted data) does not occur.
The individual time slots in each TDMA frame require overhead to allow for timing mismatch between the receiver and the transmitter or two transmitters belonging to various terminals. Thus, a guard time (during which no transmission occurs), is typically required in a time slot to prevent two transmitters with mismatched timing from transmitting simultaneously. A time slot may also incorporate overhead information to allow for circuit implementation issues such as synchronization and training (to allow a receiver to acquire the timing of a transmitted signal), or ramp up time (during which the transmitter ramps up to full transmitting power). The overhead information manifests itself as a predefined number of symbols or chips during which no terminal data may be transmitted. For example, in IS-54-B, a guard time of six symbols, a ramp up time of six symbols, and a synchronization time of 28 symbols are included as overhead information in each time slot. Thus, 40 of 162 symbols, nearly 25%, cannot be used to transmit terminal data.
Similarly, in satellite communications, a time slot, for example, 1925 chips in length, may require a beam settling time of 100 chips, a guard time of 750 chips, and a preamble (synchronization) time of 247 chips. The beam settling time is often associated with adjusting a transmitted waveform to a new coverage region, particularly when phased array antennas are used. In this example, over 36% of the bandwidth cannot be used to transmit terminal data.
In the past, attempts have been made to eliminate the synchronization portion (generally known as a preamble) of the overhead information in each time slot. The preamble of a time slot is generally used to synchronize the received signal sampling clock (using a phase locked loop or other tracking scheme) to the transmitter clock as manifested in the preamble. One approach to eliminating the preamble is disclosed in U.S. Pat. No. 4,707,841 to Yen, et al., entitled xe2x80x9cDigital Data Receiver for Preamble Free Data Transmissionxe2x80x9d. In Yen, a complicated signal processing structure is required to sample the received signal, recover symbols from the received signal, interpolate the recovered symbols, and produce digital data by decoding the recovered symbols. Thus, the well understood synchronization control associated with the preamble is eliminated. In most circumstances, this is undesirable because established communications systems require a preamble for synchronization. Thus, a time slot format that completely eliminates the preamble would be incompatible with many communication systems.
Another approach at eliminating preambles is found in U.S. Pat. No. 4,466,108 to Rhodes, entitled xe2x80x9cTDMA/PSK Carrier Synchronization Without Preamblexe2x80x9d. In Rhodes, an additional reference waveform, a xe2x80x9cpseudo-coherent referencexe2x80x9d, is required that has the same frequency as the carrier used to transmit the symbols. The reference waveform is controlled by a voltage controlled oscillator and is directly connected to a mixer for demodulation of the received waveform. Thus, no preamble is needed in the system disclosed by Rhodes. Rhodes therefore repeats the shortcoming of Yen with respect to established systems requiring a preamble.
Furthermore, neither Rhodes nor Yen suggest eliminating other types of overhead information in a TDMA frame. As noted above, each piece of overhead information in a TDMA frame lowers the transmission efficiency, inasmuch as time and power is used for purposes other than user information.
A need has long existed in the industry for a more efficient method of transmitting information in TDMA format.
It is an object of the present invention to increase the amount of transmitted user information in TDMA time slots.
It is an object of the present invention to eliminate or minimize overhead information, including preambles, in TDMA time slots.
It is another object of the present invention to periodically reinsert preambles in TDMA time slots to reestablish timing synchronization.
It is yet another object of the present invention to allow a terminal to transmit more data in multiple sequential time slots than is typically possible in individual separated time slots.
The present invention provides a method for packet concatenation in TDMA transmission. The method includes the steps of transmitting from a terminal, in a first time slot, overhead information and symbols representing terminal data. Additionally, the terminal transmits, in a subsequent time slot, only a subset of the previously transmitted overhead information and additional symbols representing terminal data. The additional symbols are transmitted in place of the subset of overhead information eliminated in the subsequent time slots. The terminal also reinserts a subset of the eliminated overhead information in a further time slot transmitted after the subsequent time slots. The further time slot may, for example, reinsert a preamble that was eliminated in the subsequent time slots.
The terminal data may include any type of information, for example, encoded voice samples for telephony users or digital fax or modem data. The overhead information may include, for example, a synchronization preamble, guard time, ramp time, or beam settling time. It is noted that overhead information generally refers to transmission of symbols, or transmission free time (for example, guard time), used for other than terminal data. In one or more subsequent time slots, the first user does not repeat certain portions of the overhead information (for example, guard time may be eliminated). Thus, only a subset of overhead information (which is, in many cases, no overhead information) is repeated in the subsequent time slot. The subsequent time slot may be a time slot consecutive with the first time slot.